Active-mode NPN transistors in circuits

The structure and use of NPN transistor. Arrow according to representation.

The diagram opposite is the representation of the NPN transistor connected to two voltage sources. To make transistor conduct appreciable current (on order of 1 mA) from C to E, V_BE should be above a minimum value at times referred to as cut-in voltage. The cut-in voltage is usually around 600 mV for silicon BJTs at the  room temperature but can be different depending on type of transistor and its  biasing. This applied voltage causes lower P-N junction to 'turn-on' allowing the flow of electrons from emitter into the base. In active mode, the electric field prevailing between base and collector (cerated by V_CE) will cause the majority of these electrons to cross upper P-N junction into the collector to form the collector current I_C. The remainder of electrons recombines with the holes, the majority carriers in base, making a current through the base connection to form base current, I_B. As shown in diagram, emitter current, I_E, is total transistor current, which is sum of the other terminal currents (i.e., I_E = I_B + I_C ).

In diagram, the arrows representing current point in the direction of  conventional current -flow of the electrons is in opposite direction of the arrows cause the electrons have negative electric charge. In the active mode, ratio of collector current to base current is called as DC current gain. This gain is nearly 100 or more, but robust circuit designs do not depend on the exact value (for instance see  op-amp). The value of this gain for DC signals is referred as h_FE, and the value of this gain for AC signals is referred to as h_fe. But, when there is no particular frequency range of interest, the symbol β is used.

It  should  be  noted  that  the  emitter  current  is  related  to  V_BE  exponentially.  At  the  room temperature, an increase in V_BE  by about 60 mV increases the emitter current by the factor of 10. Because the base current is roughly proportional to collector and emitter currents, they vary in the same manner.

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